because they have a fitness advantage due to exploitation of the exoprotease production
of wild-type cells. Similarly, when mice with burned skin or with chronic wounds are infected
with Pseudomonas aeruginosa, quorum sensing mutants act as cheats and invade the bacterial
population within days. In waxmoth larvae infections, a similar pattern of sociality
for Staphylococcus aureus can be observed. Quorum sensing mutants are less fit than their
wild-type counterparts in monoculture infections but demonstrate social cheating when in mixed
infection. Such results help us to explain why quorum sensing mutants often arise in
clinical infections even though such loss-of-function mutations might appear to be detrimental to
fitness. Furthermore, mixed infections of wild type cells with cheats, show a reduced
virulence, because cheats do not make tissue damaging toxins. This could be used to our
advantage because cheats could be introduced into infections to both reduce virulence and
buy time for the immune system to deal with the infection. This has resulted in the term
‘cheatobiotics’, but a lot more research is need in this area.
How can cooperation be maintained in the light of cheating?
A key question in the social evolution field is how are social behaviours maintained given
that cheats enjoy large fitness advantages in mixed populations? One of the most studied
ideas is ‘inclusive fitness’. This is the idea that an individual maximises its
inclusive fitness, not just by maximising its own reproduction, but the reproduction
of its genes even if they happen to be in other individuals. So by helping a relative
reproduce, this still increases inclusive fitness. This theory was developed by the
evolutionary biologist William Hamilton in the 1960s, and has since been termed ‘kin
selection’. A classic example of this can be found in insects. Many social insects such
as bees and ants, have sterile worker castes that completely forgo their own reproduction
in favour of that of the queen. How is this altruism inherited if the altruists never
reproduce? The behaviour can be inherited if the benefits accrue to individuals which
share the genes for altruism. This is the case in insects, but how does this manifest
in bacterial populations? Selection for cooperative traits in microbes can be maximized when relatedness